The Sky Eye is just another tool used to measure sunlight. It contains a photo-resistor that changes resistance as the intensity of sunlight changes. This photo-resistor is connected in series with a standard resistor to form a voltage divider circuit so that the output voltage increases as the intensity of sunlight increases. A PVC cap provides some cosine correction to better approximate the radiant energy available. The PVC material as well as the shrink sleeve stretched over the PVC cap diminishes light energy and brings the light measuring range within comprehensive limits.
At this time the Sky Eye (in conjunction with a +5 volt supply) is designed to put out mill volt readings that correspond to sunlight power measurements expressed as Watts/m2. Sky Eye Readings may vary a little from Pyranometers readings but Sky Eye readings still provide a valuable aproximation of radiant energy availability.
The electronic schematic for the sky eye is illustrated below, but remember the housing for the photo-resistor is also important.
If you need a more scientific approximation of insolation over the entire range of light intensity you may require a Pyranometers although I have found through my research that radiant energy readings below 30% have negligible value... HOWEVER this is something to keep in mind.